A Linear Variable Differential Transformer (LVDT) is a type of electrical transformer used for measuring linear (i.e. translational) displacement. Linear variable differential transformers, as illustrated in FIG. 1 (prior art), are often used as physical sensing elements in electronic circuits to provide an electrical measurement of small physical displacements, such as those produced by linear movements or pressure changes.
Early LVDT pressure transducers included mechanical components, such as an aneroid or a Bourdon tube, as illustrated in FIG. 2. The pressure applied to the aneroid caused a displacement of the core of the LVDT, thereby generating an output ratio R proportional with the applied pressure. The pressure was then determined by measuring the two output voltages V1 and V2 of the LVDT and calculating the ratio R. These LVDT pressure transducers had significant shortcomings, however, primarily due to the mechanical nature of their components. These shortcomings typically included poor accuracy, poor stability, sensitivity to vibration and shocks, and large hysteresis, i.e., sticking, due to the mechanical friction of the core. As a result, they were subject to transducer failure and erroneous outputs, particularly when subjected to high-vibration environments and acceleration forces. Therefore, while conventional mechanical LVDTs are still commonly used, it is desirable to replace them with improved LVDT-type sensing circuits that are less prone to transducer failure and erroneous output.
Solid-state implementations of LVDT-type pressure transducers have been introduced to obviate many of the shortcomings of their mechanical counterparts. One solid-state LVDT-type transducer uses a piezoresistive bridge for pressure sensing and an electronic circuit to generate LVDT-type output, as described in U.S. Pat. No. 5,398,194 (“ELECTRONIC SENSING CIRCUIT USING PIEZORESISTORS,” issued to Amnon Brosh et al. on Mar. 14, 1995 and assigned to Kulite Semiconductor Products, Inc., Leonia, N.J.). FIG. 3 provides a schematic of an implementation of one such solid-state circuit. While such solid-state traducers eliminate many of the shortcomings of mechanical LVDT-type pressure sensors, they have drawbacks of their own. For example, it has been observed that these solid-state transducers require a transformer to function properly. Disadvantageously, however, transformers are large, heavy, and expensive, often costing more than the remaining transducer components combined. Thus the cost and size are often incompatible with desired uses.
Therefore a need exists for an improved replacement LVDT-type pressure transducer that obviates the need for undesirable electro-mechanical components or transformers. It is to this need that the present invention addresses.